Definition
A dimensionless number that represents the drag characteristics of an airfoil or aircraft at a given angle of attack. It is used in the drag equation, where total drag equals the coefficient of drag multiplied by dynamic pressure and wing area. CD changes with angle of attack and configuration (flaps, gear, spoilers).
Plain English
A number that tells you how much drag a wing or aircraft produces for its size and shape at a particular angle. Higher number, more drag.
Context Anchor
Seen in aerodynamics discussions, especially when comparing lift, drag, airfoil shape, angle of attack, and aircraft performance.
Derivation
Coefficient' comes from Latin roots meaning 'working together' — a number that works alongside other values in an equation. 'Drag' is the resisting force opposing motion through the air. So the coefficient of drag is the multiplier that, when combined with speed and wing area, produces the actual drag force.
Why Pilots Care
It directly affects how much power is needed and the aircraft's efficiency in flight.
Grounding Statement
If two airplanes have the same speed, air density, and wing area, the one with the higher coefficient of drag will produce more drag.
Intuition Check
Drag here does not mean pulling something along the ground; it means the air’s resisting force on the airplane. CD is not the drag force itself; it is the number that helps determine how much drag is produced.
Example Sentence 1
Extending the flaps increased the coefficient of drag, which is why the airplane decelerated quickly on final approach.
Example Sentence 2
A high coefficient of drag reduces the aircraft's glide ratio, limiting how far it can travel without power.